You get better at it the more you do. A tip is also to actually change a mathematical exposition into a form you better understand (e.g. by writing it in a diff

You get better at it the more you do. A tip is also to actually change a mathematical exposition into a form you better understand (e.g. by writing it in a different notation and/or expanding it out in words to make the existing notation less dense). Basically convert the presentation into the way you would personally like to see it. If you do this enough, the process becomes easier and the original notation becomes easier to understand. But it takes a lot of time and patience (as I'm sure it took for you understand undocumented code did as well).